Method for the production of isobutene from commercial methyl tert-butyl ether

ABSTRACT

The invention relates to a process for preparing high-purity isobutene from industrial methyl tert-butyl ether (MTBE) and the economical utilization of the secondary streams.

The invention relates to a process for preparing high-purity isobutenefrom industrial methyl tert-butyl ether (MTBE) and the economicalutilization of the secondary streams.

Isobutene is a starting material for the production of butyl rubber,polyisobutylene, isobutene oligomers, branched C₅-aldehydes andC₅-carboxylic acids. Furthermore, it is used as alkylating agent andintermediate for the production of peroxides.

Isobutene can be obtained by dehydrogenation of isobutane. However,sufficiently large amounts of pure isobutane are not available.

In industrial streams, for example, in the C₄ fraction from a steamcracker or an FCC unit, isobutene is present together with saturated andunsaturated hydrocarbons. Owing to the small boiling point difference orthe very low separation factor between isobutene and 1-butene, isobutenecannot be separated economically from these mixtures by distillation.

The isobutene can be separated off from these C₄ fractions in variousways, depending on which further olefins are to be obtained. The firststep which is common to all work-up methods is removal of the major partof the butadiene and other multiply unsaturated hydrocarbons. Ifbutadiene can readily be marketed or there is an in-house demand, it isseparated off by extraction or extractive distillation. Otherwise, it isselectively hydrogenated to linear butenes down to a residualconcentration of about 2000 ppm by mass. In both cases, the stream whichremains is a hydrocarbon mixture (known as raffinate I or hydrogenatedcracking C₄) which comprises the saturated hydrocarbons n-butane andisobutane together with the olefins isobutene, 1-butene and 2-butenes(cis and trans).

If 2-butene or a mixture of linear butenes having a high 2-butenecontent is to be obtained in addition to isobutene, the abovementionedmixture, which typically contains not more than 1% of butadiene (C₄stream from FCC, raffinate I or hydrogenated cracking C₄), ishydrogenated and hydroisomerized, i.e. residual butadiene still presentis selectively hydrogenated down to a residual content of below 5 ppmand 1-butene is isomerized to the 2-butenes. The equilibrium ratio of1-butene to the two 2-butenes is, for example, about 1/17, i.e. far onthe side of the 2-butenes, at 80° C. Owing to the small boiling pointdifferences, distillation of the hydroisomerization mixture gives only amixture of isobutene, 1-butene and isobutane as top product, from whichthe isobutane can be separated off by distillation. The bottom productobtained is an isobutene-free mixture comprising 2-butenes. Even if thehydroisomerization is carried out in a reactive distillation column, nocompletely 1-butene-free isobutene is obtained, as described, forexample, in EP 0 922 018. This isobutene is therefore not suitable forsome applications.

Isobutene can be separated off from a C₄-olefin mixture via the stepsselective conversion into a derivative, separation of the derivativefrom the remaining hydrocarbon mixture and dissociation of thederivative.

Isobutene can easily be converted into derivatives by means of water oralcohols. The reaction of isobutene-containing streams with water givestert-butanol (TBA) which can easily be redissociated into isobutene andwater. The main disadvantage of this separation process is the TBAsynthesis, which, owing to the low solubility of water inC₄-hydrocarbons, gives only low space-time yields.

The addition of methanol onto isobutene in C₄-hydrocarbon streams toform MTBE proceeds substantially more quickly than the addition ofwater. Industrial MTBE is a valued fuel component for four-strokeengines for the purpose of increasing the octane number. Owing to theease with which it can be prepared and its large market volume, it is aninexpensive precursor for pure isobutene.

For this reason, the industrial procedure is usually to react anisobutene-containing C₄ fraction with methanol to form MTBE, purify thelatter and redissociate it into isobutene and methanol.

This process has the disadvantage that it is difficult to prepareisobutene having a purity of greater than 99.9%. Industrial MTBE (fuelgrade) further comprises C₄- and C₅-hydrocarbons, C₄-oligomers (C₈-,C₁₂-hydrocarbons), 2-methoxybutane (MSBE), methanol and TBA. Thesematerials and their downstream products and also other by-productsformed from MTBE during the dissociation can contaminate the targetproduct isobutene.

Integrated processes for preparing high-purity isobutene from C₄-streamsvia the preparation of MTBE and its dissociation are known anddescribed, for example, in U.S. Pat. No. 5,567,860. Here,isobutene-containing C₄ streams are firstly etherified with methanol togive, depending on the conversion, a mixture of MTBE, MSBE, unreactedC₄-hydrocarbons, methanol, water, DME, C₄ oligomers and also C₃- andC₅-hydrocarbons as contaminants in the C₄ stream. This product mixtureis fractionally distilled to give a low-boiling fraction comprising theC₃-, C₄- and C₅-hydrocarbons, methanol and DME and a high-boilingfraction comprising C₄ oligomers. MTBE and MSBE are obtained in a sidestream taken from the column and are then passed to the acid-catalyzeddissociation. The dissociation reaction accordingly gives isobutene,n-butene and methanol as main constituents together with unreacted MTBEand MSBE. This product mixture is in turn purified by distillation, withthe C₄/methanol azeotrope comprising isobutene and n-butene and DMEbeing taken off as low-boiling fraction. To obtain the target product,viz. high-purity isobutene, this fraction has to be purified by means ofat least one water scrub and a distillation. The high-boiling fractionobtained from the dissociation reaction (MTBE, methanol, MSBE) isfractionated to give methanol as high boiler and an azeotrope ofmethanol, MTBE and MSBE as low boiler. These fractions are eachrecirculated to a point upstream of the etherification stage or thedissociation stage.

This process is complicated in that the target product isobutene has tobe freed of the accompanying substances in the C₄ feed stream andunreacted reaction products or by-products from the etherification anddissociation reactions in a plurality of columns and scrubbing stages.Furthermore, an integrated process should make it possible for unreactedmaterials such as MTBE or isobutene-containing C₄ streams to beseparated off in a simple fashion and be recirculated to the appropriatereaction stages. In the ideal case, isobutene-containing C₄ hydrocarbonsand recovered methanol would be separated off at a point in the processand reused for the preparation of MTBE. On the other hand, unreactedMTBE should be obtained as a separate stream and be recirculated to theether dissociation reaction.

It is therefore an object of the present invention to provide a processfor preparing isobutene from MTBE which can be operated using very fewseparation stages and with few recycle streams.

The present invention accordingly provides a process for preparingisobutene by acid-catalyzed dissociation of methyl tert-butyl ether(MTBE), which comprises

-   -   fractionating a feed mixture comprising MTBE, C₄- and        C₅-hydrocarbons, methanol, methyl sec-butyl ether, TBA and C₄        oligomers to give

-   a) a fraction a) comprising MTBE, MSBE, TBA and C₄ oligomers and

-   b) a fraction b) comprising C₄- and C₅-hydrocarbons, MTBE and    methanol,

-   c) dissociating the MTBE present in the fraction a) into methanol    and isobutene and

-   d) separating off an isobutene-containing stream from the    dissociation product from c) and recirculating the remainder to the    feed mixture.

The MSBE present in the feed mixture passes unspecifically into the twofractions a) and b), but is removed in an advantageous fashion by meansof a bleed stream from the fraction a).

The process of the invention can easily be linked to an existing MTBEplant, so that the recycle streams of methanol and C₄-hydrocarbons canbe reused for the preparation of MTBE. It is also possible to useindustrial MTBE of fuel quality or with even lower specifications.

Compared to the prior art, the process of the invention achieves aparticularly elegant removal of impurities present in the feed mixtureand streams to be recirculated to other process stages. Thus, methanoland low-boiling impurities in the feed mixture are separated off in thefirst distillation stage prior to the dissociation reaction. This makesit possible for the isobutene obtained in the dissociation reaction tobe separated off efficiently, since interfering accompanying materialshave already been separated off. The recirculation of the methanolobtained in the reaction to upstream of the dissociation reaction, ormore precisely upstream of the first distillation stage, results inefficient circulation of the methanol with simultaneous removal ofby-products such as DME, C₄ oligomers, TBA or MSBE in an MTBE synthesispreceding the process of the invention.

A block diagram of a plant in which the process of the invention can becarried out is shown in FIG. 1. MTBE (fuel grade) (1) together with thebottom product (11) from column (9) is fed into the column (2). Amixture of MTBE, methanol and C₄- and C₅-hydrocarbons is taken off astop product (3). A part (6) of the bottom product (4) from the column(2), which comprises predominantly MTBE, is separated off to dischargehigh boilers (TBA, diisobutene, MSBE). The other part (5) is fed to thedissociation reactor (7). The reaction mixture (8) is fractionated inthe distillation column (9). The top product (10) obtained is anisobutene which may further comprise methanol, dimethyl ether and water.The optional work-up of this crude isobutene to give high-purityisobutene is not shown in FIG. 1. The bottom product (11) from thecolumn (9), which comprises undissociated MTBE, part of the methanolformed in the dissociation and high boilers, is recirculated to thecolumn (2). In place of the reactor (7) and the column (9), it is alsopossible to use one or more reactive distillation columns. All or partof the stream (3) can be conveyed via line (12) to an optionaletherification stage (13). Here, MTBE is prepared from anisobutene-containing C₄-hydrocarbon stream (14), fresh methanol (15) andthe recirculated methanol (12). Stream (16) serves to bleed offunreacted components from the isobutene-containing C₄-hydrocarbon stream(e.g. n-butene and aliphatic constituents).

The feed to the process of the invention can be industrial MTBE of fuelgrade. This typically comprises 98% by mass of MTBE together with about0.5% by mass of C₄- to C₅-hydrocarbons, about 1% by mass of methanol,about 500 ppm by mass of water and 2-methoxybutane. Preference is givento using an industrial MTBE having a 2-methoxybutane content of lessthan 2500 ppm by mass, whose preparation is described, for example, inDE 101 02 082.

It is also possible to use MTBE grades having a methanol contentsignificantly higher than 1% by mass, e.g. MTBE/methanol mixtures havinga ratio of 80:20, 90:10 or 95:5 can be processed without problems. Thesemixtures can naturally further comprise the accompanying materialsmentioned above in an amount of ≦3% by weight.

In the process of the invention, the C₄- and C₅-hydrocarbons in the MTBEare removed by distillation together with the MTBE-methanol minimumazeotrope. This gives a distillate comprising MTBE, methanol and C₄- andC₅-hydrocarbons. This mixture is advantageously fed to the synthesisstage of an MTBE plant.

As a result of the recirculation of the bottom product (11) from thedistillation column (9), the C₄- and C₅-hydrocarbons are removed fromthe MTBE feed in the column (2) and the major part of the methanolformed in the MTBE dissociation is also separated off in this column.

It has in practice been found to be useful for the distillation columnupstream of the dissocation reactor ((2) in FIG. 1) to have from 10 to60 theoretical plates, in particular from 20 to 40 theoretical plates,of which from 10 to 30 are in the enrichment section and from 10 to 30are in the stripping section.

The column is advantageously provided with internals such as trays,random packing or ordered packing. The fractionation in this column canbe carried out at atmospheric pressure or under superatmosphericpressure. Since the proportion of MTBE in the MTBE/methanol azeotropedecreases with increasing pressure in the pressure range from 1 to 25bar and as little MTBE as possible should be separated off together withthe methanol, the distillation is preferably carried out undersuperatmospheric pressure, in particular in the pressure range from 5 to25 bar, very particularly preferably in the pressure range from 8 to 20bar.

The reflux ratio in the column is from 1 to 10, in particular from 2 to7.

The bottom product from the column (2) upstream of the dissociationreactor (step a) (FIG. 1) preferably has a content of C₄- andC₅-hydrocarbons of less than 250 ppm by mass. It contains a small amountof methanol and also TBA, 2-methoxybutane and diisobutene as highboilers.

To separate off C₄- and C₅-hydrocarbons, their oligomers and methanolfrom the dissociation together and at the same time prevent accumulationof high boilers in the process, a part (6) of the bottom product (4)from the column (2) can be bled off continuously. The discharge of theC₄ oligomers from the fraction a) of the process of the invention can becarried out either by means of a bleed stream or by means of a furtherdistillation stage, e.g. as bottom product. One possible use for thebleed stream is work-up by distillation to produce high-purity MTBE. Forthis it is necessary to reduce the methanol content of the bottoms (4)from the column (2) to 50 ppm by mass, which can be achieved by takingoff an increased amount of distillate from the column (2).

The dissociation of the bottom product, which comprises predominantlyMTBE, into isobutene and methanol can be carried out over an acidcatalyst located in a fixed bed.

One group of acid catalysts which can be used in the process of theinvention are solid ion exchange resins bearing sulfonic acid groups.

Suitable ion exchange resins are, for example, those which are preparedby sulfonation of phenol/aldehyde condensates or of cooligomers ofaromatic vinyl compounds. Examples of aromatic vinyl compounds forpreparing the cooligomers are: styrene, vinyltoluene, vinylnaphthalene,vinylethylbenzene, methylstyrene, vinylchlorobenzene, vinylxylene anddivinylbenzene. In particular, the cooligomers formed by reaction ofstyrene with divinylbenzene are used as intermediate for the preparationof ion exchange resins bearing sulfonic acid groups. The resins can beprepared in gel form, macroporous form or sponge form. Strong acidresins of the styrene-divinylbenzene type are sold, inter alia, underthe following trade names: Duolite C20, Duolite C26, Amberlyst A15,Amberlyst A35, Amberlyst 36, Amberlite IR-120, Amberlite 200, Dowex 50,Lewatit K2431, Lewatit K2441, Lewatit K2621, Lewatit K2629, LewatitK2641.

The properties of these resins, in particular the specific surface area,porosity, stability, swelling or shrinkage and exchange capacity, can bevaried by means of the production process.

If desired, it is also possible to use commercial, macroporous cationexchangers which have been modified by partial ion exchange or bythermal desulfonation.

The dissociation of MTBE is carried out in one or more reactors. When aplurality of reactors are used, these can be connected in series or inparallel or both in series and in parallel. it is possible to usevarious types of reactor, for example fixed-bed reactors orshell-and-tube reactors or kettle reactors.

The reactor(s) is/are operated isothermally, polytropically oradiabatically, in a single pass or with an external recycle loop.

The reaction temperature in the dissociation reactor in the process ofthe invention is in the range from 60° C. to 200° C., preferably from80° C. to 120° C. When a plurality of reactors are used, thetemperatures can be identical or different.

The dissociation of MTBE can be carried out in the liquid phase overacid ion exchange resins as described, for example, in DE 3 509 292 orDE 3 610 704 or over acidic aluminum oxides as disclosed, for example,in DD 240 739. In the latter case, the reaction conditions (167° C. and1 bar or 297° C. and 10 bar) are selected so that the dissociation ofMTBE can also proceed in the gas/liquid region. However, in the case ofdissociation processes carried out in the pure liquid phase, it has tobe noted that high MTBE conversions cannot be achieved in a single passbecause of the position of the thermodynamic equilibrium. If pure MTBEis used in a dissociation reaction which is preferably to proceed at100° C., an equilibrium conversion of about 15 mol % is obtained on thebasis of the thermodynamics. A problem in the dissociation in the liquidphase is the isobutene which is dissolved in the homogeneous liquidphase and can undergo further reactions. The most important furtherreactions of this type are acid-catalyzed dimerization andoligomerization. For this reason, undesirable C₈ and C₁₂ components arefound together with the desired target product isobutene. Theundesirable C₈ molecules are 2,4,4-trimethyl-1-pentene and2,4,4-trimethyl-2-pentene. High reaction temperatures also favor theundesirable secondary reaction of methanol to form dimethyl ether (DME).The formation of dimethyl ether not only leads to a loss of methanol butalso increases the difficulty of purifying the isobutene.

The MTBE dissociation reaction can also be carried out in a reactiondistillation column, as disclosed in EP 0 302 336 or DE 4 322 712. EP 0302 336 describes the elimination of methanol from MTBE over an acid ionexchange resin which is located in the bottom of the column. Thedissociation of the ether in this case takes place in the bottom of thecolumn, i.e. the catalyst is continually surrounded by a mixture ofether, olefin and alcohol. This is a disadvantage for the preparation ofisobutene, since higher oligomers of isobutene are easily formed atrelatively high temperatures under the acidic conditions. In addition,the acid centers of the catalyst are occupied by methanol, which leadsto undesirable formation of dimethyl ether. For this reason, a differentroute is taken in DE 4 322 712. There, the tertiary ether is fed into areaction distillation column above the reaction zone, and the enrichmentsection of the column serves for purifying the isobutene while methanolis separated from the MTBE-methanol azeotrope in the stripping sectionof the column. The azeotrope goes back into the reaction zone. As acidcatalyst, use is made of a sulfated titanium dioxide extrudate. DE 10020 943 discloses an alternative process in which the ether to bedissociated (e.g. MTBE) is introduced into a reactive distillationcolumn below the reaction zone. The actual dissociation takes place inan azeotrope of the ether with the corresponding alcohol.

If the dissocation reactor (7) and the column (9) are configured as areactive distillation in the process of the invention, preference isgiven to using structured catalytic multipurpose packing as described,for example, in U.S. Pat. No. 5 348 710, EP 0 950 433, EP 0 428 265, EP433 222. Such structured packing which can be used for the purposes ofthe process of the invention is, for example, commercially available asKatapak® from Sulzer AG, Katamax® from Koch-Glitsch or Multipak® fromMontz GmbH. These types of packing are customarily constructed of sheetmetal, preferably mild steel, stainless steel, Hastelloy, copper oraluminum, or structured sheets of mesh.

The dissociation mixture comprising unreacted MTBE, methanol, isobutene,low boilers and high boilers is separated in a column ((9) in FIG. 1)into an isobutene-containing top product and a bottom product comprisingthe unreacted MTBE and the major part of the methanol from thedissociation.

In a further process variant, it is also possible to fractionate thebottom product from the column (9) (FIG. 1) in an additional column (notshown in FIG. 1) to give an MTBE-rich bottom product and a top productcomprising mainly an MTBE/methanol azeotrope. Part of this bottomproduct can be bled off to remove high boilers. The other part isreturned to the dissociation stage.

If desired, the bottom product from column (9) or the reactivedistillation column can be fed to the synthesis stage of an MTBE plant.

The isobutene which has been separated off from the reaction mixture bydistillation comprises methanol, water and dimethyl ether. If desired,methanol is removed therefrom by extraction with water using methodsknown per se.

The isobutene-containing stream can be fractionated in a purificationcolumn to give a bottom product consisting of high-purity isobutene anda top product comprising isobutene, low-volatility by-products andpossibly water. This purification column can likewise be preceded by awater scrub to remove methanol.

It is also possible to remove the water present in theisobutene-containing stream which has been separated off (in particularafter a scrubbing stage) by means of a decanter. In the decanter, a feedstream comprising isobutene, DME and water is separated into a heavy,aqueous phase and a light, organic phase comprising isobutene and DME.

In the process of the invention, this is preferably carried out using acolumn provided with a decanter for the removal of water located in theside stream from the colum. Locating the decanter in the side streamminimizes the isobutene losses. It is also possible to install thedecanter at the top of the column.

FIG. 2 schematically shows such a procedure. The isobutene-containingstream (10) obtained, for example, as shown in FIG. 1 is scrubbed withwater (16) in the extractor (15). This isobutene stream (17) whichfurther comprises dimethyl ether and water is fed into the column (18)from which dimethyl ether is taken off as top product (19) andhigh-purity isobutene is taken off as bottom product (20). A liquid sidestream (21) is taken off below the feed point and is separated in thedecanter (22) into an aqueous phase (23) and an organic phase (24) whichhas been depleted in water. Water (23) is taken off and the organicphase (24) is recirculated to the column.

The pure isobutene column (18) preferably has from 25 to 50 theoreticalplates, in particular 30-40 theoretical plates. The isobutene to bepurified is fed into the 15th to 30th theoretical plate, in particularinto the 18th to 24th theoretical plate, in each case counted from thebottom. At a point located from two to five theoretical plates below thefeed point, the entire condensate of this theoretical plate is taken offand passed to the decanter. After the water has been separated off bymechanical means, the organic phase is returned to the column at a pointwhich is from one to two theoretical plates further down.

In a particular embodiment of the water removal, the decanter (22) isconfigured as a decantation tray within the column, e.g. in the topsection. In this case, only the aqueous phase is obtained as sidestream.

The distillation can be carried out at pressures of from 8 to 20 bar, inparticular from 8 to 12 bar. The distillation temperatures are dependenton the pressure. For example, the temperature at the top at 9 bar isabout 40° C.

The isobutene obtained by the process of the invention has a purity offrom 99.90 to 99.98% by mass. in particular from 99.94 to 99.98% bymass, very particularly preferably from 99.96 to 99.98% by mass.

In the process of the invention, internals comprising trays, randompacking or ordered packing can be used for the distillation (column (2),(9) in FIG. 1; column (18) in FIG. 2). In the case of column trays, thefollowing types are used: trays having holes or slits in the baseplate,trays having necks or chimneys covered by bells, caps or hoods, trayshaving holes in the baseplate which are covered by movable valves. It isalso possible to use disordered beds of various packing elements. Theycan be made of virtually all materials, e.g. steel, stainless steel,copper, carbon, stoneware, porcelain, glass, plastics, etc., and canhave various shapes, e.g. spheres, rings having smooth or profiledsurfaces, rings having internal webs or openings through the wall, wiremesh rings, saddles and spirals. Packing having a regular geometry canbe made of, for example, metal sheets or meslhes. Examples of suchpacking are Sulzer mesh packing BX made of metal or plastic, Sulzerlamellar packing Mellapack made of sheet metal, structured packing fromSulzer (Optiflow), Montz (BSH) and Kuhni (Rombopack).

The following examples illustrate the invention without restricting itsscope which is defined by the description and the claims.

EXAMPLE 1

Dissociation of MTBE With Isobutene and Methanol Being Separated Off

The dissociation of MTBE and the separation of the isobutene producedand the methanol from the unreacted MTBE were carried out in a plant asshown in FIG. 1 but without the MTBE synthesis stage (13). TheMTBE-methanol azeotrope and the C₄- and C₅-hydrocarbon components wereseparated off using a column (2) which was packed with Sulzer BX meshpacking and had 30 theoretical plates. The enrichment section had aninternal diameter of 50 mm and 15 theoretical plates and the strippingsection had an internal diameter of 80 mm and likewise 15 theoreticalplates. The isobutene was separated off in a column (9) having aninternal diameter of 50 mm which was likewise equipped with Sulzer BXmesh packing and had 35 theoretical plates. The dissociation of MTBE wascarried out using a tube reactor (7) having an internal diameter of 21mm and a length of 160 mm. As catalyst, use was made of a commercial ionexchange resin from Bayer, Lewatit K2621. The tube reactor was operatedin a thermostated oil bath at 100° C.

The operating parameters of the two columns and the reactor were asfollows: Azeotrope Isobutene Dissociation column column reactor (2) (9)(7) Pressure bar  10  5 Pressure bar 20 Temperature Temperature ° C.100  Top ° C. 128 42 MTBE conversion % 16 Feed ° C. 135 84 Bottom ° C.148 106  Feed plate from the bottom  15 20 Reflux ratio kg/kg  4  3

The amounts and compositions of the individual streams are shown in thefollowing tables. industrial MTBE (Driveron®) was used as feed. MTBEColumn (2) Reactor Bleed Reactor Column (9) feed distillate bottoms feedstream product distillate bottoms (1) (3) (4) (5) (6) (8) (10) (11) Massflow kg/h 8.0 3.6 22.5 20.2 2.2 20.2 2.1 18.1 Proportion by massDimethyl ether kg/kg 0.00000 0.00000 0.00000 0.00000 0.00000 0.000050.00048 0.00000 Isobutane kg/kg 0.00025 0.00055 0.00000 0.00000 0.000000.00000 0.00000 0.00000 Isobutene kg/kg 0.00000 0.00005 0.00000 0.000000.00000 0.10185 0.96873 0.00001 1-Butene kg/kg 0.00010 0.00022 0.000000.00000 0.00000 0.00000 0.00000 0.00000 n-Butane kg/kg 0.00010 0.000220.00000 0.00000 0.00000 0.00000 0.00000 0.00000 trans-2-butene kg/kg0.00025 0.00055 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000cis-2-butene kg/kg 0.00030 0.00066 0.00000 0.00000 0.00000 0.000000.00000 0.00000 C5-hydrocarbons kg/kg 0.00200 0.00437 0.00007 0.000070.00007 0.00007 0.00000 0.00008 MTBE kg/kg 0.97840 0.67047 0.973460.97346 0.97346 0.81498 0.00000 0.91072 2-Methoxybutane kg/kg 0.003000.00284 0.00610 0.00610 0.00610 0.00610 0.00000 0.00682 Methanol kg/kg0.00600 0.31706 0.00005 0.00005 0.00005 0.05759 0.03036 0.06079tert-butanol kg/kg 0.00800 0.00000 0.00945 0.00945 0.00945 0.007330.00000 0.00820 Water kg/kg 0.00010 0.00295 0.00000 0.00000 0.000000.00053 0.00043 0.00054 Diisobutene kg/kg 0.00150 0.00005 0.010870.01087 0.01087 0.01150 0.00000 0.01285

EXAMPLE 2

Removal of DME and Water From the Isobutene Using a Decanter at the Topof the Column

The purification of the isobutene by removal of dimethyl ether and waterwas carried out as shown in FIG. 3 in a column having a diameter of 50mm. The column was equipped with Sulzer BX mesh packing and had 35theoretical plates. The decanter (22) from which an aqueous phase (23)was taken off was located at the top of the column (18). The feed to theplant came from an MTBE dissociation (e.g. as shown in FIG. 1) withdownstream removal of methanol by extraction with water.

Operating parameters of the column: DME column (18) Pressure bar  9Temperature Top ° C. 57 Feed ° C. 60 Bottoms ° C. 67 Feed plate from thebottom 20 Reflux ratio kg/kg 43

Flow data: Distillate Bottoms Stream taken Rumback Feed from from Feedto off from from column column column decanter decanter decanter (17)(19) (20) (21) (23) (24) Mass flow Kg/h 6.000 0.199 5.798 8.736 0.0028.733 Proportion by mass Dimethyl ether kg/kg 0.01000 0.29995 0.000030.20378 0.04311 0.20382 Isobutene kg/kg 0.98930 0.69278 0.99987 0.788590.00226 0.78879 C5-hydrocarbons kg/kg 0.00010 0.00000 0.00010 0.000000.00000 0.00000 Methanol kg/kg 0.00001 0.00013 0.00000 0.00017 0.001900.00017 Water kg/kg 0.00060 0.00714 0.00000 0.00746 0.95273 0.00721

This experiment, in which the operation parameters had been optimized interms of isobutene purity and yield, showed that the maximum dimethylether concentration in the distillate is limited to about 30% by weight.As a result, about 2.5% of the isobutene is lost via the distillatestream. This amount of loss cannot be reduced using this decanterarrangement.

EXAMPLE 3

Removal of DME and Water From the Isobutene Using a Decanter Located atthe Side

These experiments were carried out as shown in FIG. 2 using the samecolumn as in Example 2, but the decanter was located below the feedplate.

Operating parameters of the column: DME column (18) Pressure bar  9Temperature Top ° C. 41 Feed ° C. 60 Bottom ° C. 67 Feed (17) to 21 sidestream (21) from 15 recycle (24) to 14 *from the bottom Reflux ratiokg/kg 170 

Stream data: Distillate Bottoms Stream taken Rumback Feed from from Feedto off from from column column column decanter decanter decanter (17)(19) (20) (21) (23) (24) Mass flow Kg/h 6.000 0.063 5.933 19.003 0.00419.000 Proportion by mass Dimethyl ether Kg/kg 0.01000 0.94949 0.000030.00556 0.00117 0.00556 Isobutene Kg/kg 0.98930 0.05051 0.99987 0.993290.00179 0.99348 C5-hydrocarbons Kg/kg 0.00010 0.00000 0.00010 0.000050.00000 0.00005 Methanol Kg/kg 0.00001 0.00000 0.00000 0.00031 0.008230.00031 Water Kg/kg 0.00060 0.00000 0.00000 0.00079 0.98881 0.00060

It can be seen from Example 3 that dimethyl ether concentrations ofgreater than 95% by weight can be achieved in the distillate by means ofthe particular arrangement of the decanter and virtually no isobutenelosses therefore occur. Thus, better economics than in Example 2 areobtained.

1. A process for preparing isobutene by acid-catalyzed dissociation ofmethyl tert-butyl ether (MTBE), said process comprising: fractionating afeed mixture comprising MTBE, C₄- and C₅-hydrocarbons, methanol, methylsec-butyl ether, TBA and C₄ oligomers to give a) a fraction a),comprising MTBE, MSBE, TBA and C₄ oligomers. and b) a fraction b),comprising C₄- and C₅-hydrocarbons, MTBE and methanol, c) dissociatingthe MTBE present in the fraction a) into methanol and isobutene. and d)dissociating the dissociation product from c) which comprises unreactedMTBE, methanol, isobutene and low boilers and high boilers in a columninto an isobutene-containing top product and a bottom product comprisingthe unreacted MTBE and the major part of the methanol from thedissociation, and recirculating the bottom product to the feed mixture.2. The process as claimed in claim 1, wherein the C₄ oligomers, MSBE andTBA are separated off from the fraction a) by means of a distillation,in which they are taken off as bottom product.
 3. The process as claimedin claim 1, wherein the C₄ oligomers, MSBE and TBA are separated offfrom the fraction a) by means of a bleed stream.
 4. The process asclaimed in claim 1, wherein the isobutene-containing stream, which hasbeen separated off from the dissociation product from c), isfractionated in a purification column to give a bottom productconsisting of pure isobutene and a top product comprising isobutene andvolatile by-products.
 5. The process as claimed in claim 1, wherein theisobutene-containing stream, which has been separated off from thedissociation product from c), is scrubbed with water, and subsequentlyfractionated in a purification column to give a bottom productconsisting of pure isobutene and a top product comprising isobutene andvolatile by-products.
 6. The process as claimed in claim 4, whereinwater present in the isobutene-containing stream is removed by means ofa decanter.
 7. The process as claimed in claim 4, wherein water presentin the isobutene-containing stream is removed by means of a decanterlocated in the top section of the purification column.
 8. The process asclaimed in claim 4, wherein water present in the isobutene-containingstream is removed by means of a decanter which is located at a the sideofftake of the purification column.
 9. The process as claimed in claim1, wherein the dissociation of step c) and the separation of theisobutene in step d) from the MTBE present in fraction a) are carriedout in a reactive distillation column.
 10. The process as claimed inclaim 2, wherein the isobutene-containing stream, which has beenseparated off from the dissociation product from c), is fractionated ina purification column to give a bottom product consisting of pureisobutene and a top product comprising isobutene and volatileby-products.
 11. The process as claimed in claim 3, wherein theisobutene-containing stream, which has been separated off from thedissociation product from c), is-fractionated in a purification columnto give a bottom product consisting of pure isobutene and a top productcomprising isobutene and volatile by-products.
 12. The process asclaimed in claim 2, wherein the isobutene-containing stream, which hasbeen separated off from the dissociation product from c), is scrubbedwith water, and subsequently fractionated in a purification column togive a bottom product consisting of pure isobutene and a top productcomprising isobutene and volatile by-products.
 13. The process asclaimed in claim 3, wherein the isobutene-containing stream, which hasbeen separated off from the dissociation product from c), is scrubbedwith water, and subsequently fractionated in a purification column togive a bottom product consisting of pure isobutene and a top productcomprising isobutene and volatile by-products.
 14. The process asclaimed in claim 5, wherein water present in the isobutene-containingstream is removed by means of a decanter.
 15. The process as claimed inclaim 5, wherein water present in the isobutene-containing stream isremoved by means of a decanter located in the top section of thepurification column.
 16. The process as claimed in claim 6, wherein thewater present in the isobutene-containing stream is removed by means ofa decanter located in the top section of the purification column. 17.The process as claimed in claim 5, wherein water present in theisobutene-containing stream is removed by means of a decanter which islocated at a side offiake of the purification column.
 18. The process asclaimed in claim 6, wherein the water present in theisobutene-containing stream is removed by means of a decanter which islocated at a side offtake of the purification column.
 19. The process asclaimed in claim 7, wherein the water present in theisobutene-containing stream is removed by means of a decanter which islocated at a side offtake of the purification column.
 20. The process asclaimed in claim 2, wherein the dissociation of step c) and theseparation of the isobutene in step d) from the MTBE present in fractiona) are carried out in a reactive distillation column.